Owing to the increasing ecological awareness, as well as the strict environmental regulations, and consequently the need to develop environmentally friendly processes, attention is currently focused on the development of “green” alternatives to mitigating corrosion. Green approaches to corrosion mitigation entail the use of substances, techniques, and methodologies that reduce or eliminate the use of/generation of feedstocks, products, byproducts, solvents, reagents, and so forth that are hazardous to human health or the environment in combating corrosion. They include the use of plant extracts, ionic liquids, biochemicals, and biodegradable organic and green inorganic inhibitors; the development of corrosion inhibitors based on waste products; protection against corrosion by corrosion byproducts, ionexchange pigments, and coatings; the development of mitigation techniques that enable the detection and prevention of the early stages of corrosion, and so forth.
Of all these methods, the use of plant extracts in corrosion inhibition studies has continued to attract patronage as evident in the papers published in this special issue. Four papers present the inhibition behaviour of plant extracts in the corrosion of metals in acidic media, and another one gives an overview of plant extracts used as corrosion inhibitors in aqueous media. It is interesting to note that in the field of corrosion inhibition we are going back to the past as a result of the fact that increasing awareness of health and ecological risks is drawing attention to finding more suitable nontoxic inhibitors, which are found mostly among the class of natural products. In the Middle Ages (i.e., from the 5th century to the 15th century), the use of plant extracts (flour, bran, yeast, a mixture of molasses and vegetable oil, and starch) for pickling of metal articles by master armourers has been reported [1, 2]. However, the available information indicates that the earliest documented use of corrosion inhibitors appears to be that described by Marangoni and Stephanelli [3], who used extracts of bran among other substances to inhibit the corrosion of iron in acids. Few years later, the first patent in corrosion inhibition was given to Baldwin in 1895 [2] who specified the use of natural plant product, molasses and vegetable oils, for pickling sheet steel in acids. In US Patent 640491 in 1900 given to Robinson and Sutherland, they used starch, a biodegradable material.
In 1930, plant extracts (dried stems, leaves, and seeds) of celandine (Chelidonium majus) and other plants were used in H2SO4 pickling baths [2]. In the 1970s and 80s, the study of plant extracts as corrosion inhibitors became more extensive.
Based on previous works in the 70s and 80s, one would have thought that there will be an upsurge in the publications and application of plant extracts as metallic corrosion inhibitors in the 90s. This was not the case, as the literature search indicated the contrary. The reason is not unconnected with the difficulties in isolating and purifying the active ingredients of the extracts as most reviewers insisted that the active principles responsible for the inhibition be identified and tested, coupled with the mild enforcement of the laws on the use of ecofriendly inhibitors.
At the inception of this new millennium, various research groups showed an increased interest in the use of natural products as corrosion inhibitor resulting in enormous data on plant extract as corrosion inhibitors. The reason for this uninterrupted interest can undoubtedly be ascribed to an increased awareness of the environmental requirements that is currently imposed on the development of cleaner chemical inhibitors, of the health risks associated with the use of unsafe and toxic inorganic inhibitors, and of the great contribution that these data can give to developing eco-friendly corrosion inhibitors. This clearly shows that the era of green inhibitors is here.
Green approaches to corrosion mitigation also involve the use of green chromate-free organic inhibitors as explored by L. A. Hernandez-Alvarado et al. and the development of techniques that enable the detection and prevention of corrosion as presented in this special issue.
Peter C. Okafor
Eno E. Ebenso
Ali Y. El-Etre
Mumtaz Ahmad Quraishi